An antifouling coating composition comprising a film-forming, thermoplastic or thermosetting polymer, an organic liquid diluent, an antifouling agent and other optional additives has been widely used as ship bottom paint, marine structure paint, fishnet paint and other similar use paints for the control of attachment of marine organisms or the like. However, in such coating compositions, since the film-forming polymer is usually of unhydrolyzable nature and the antifouling effect is entirely dependent upon the amount of antifouling agent dissolved out of the coating, there is a serious problem such that the desired antifouling effect can not last long. This is because, at the beginning stage in where the antifouling agent is dissolved out at the surface area of the coating, a higher antifouling effect can be realized therewith, and however, as it comes to the stage wherein the antifouling agent present in the surface area has already been thoroughly consumed and the agent contained in the inside of the coating is dissolved out as a consequence of diffusion due to the presence of concentration gradient, the desired antifouling effect is markedly decreased. Furthermore, after said dissolution, the coating will get skeleton structure, which in turn causes, in the case of ship bottom paint, a marked decrease in ship's speed and increase in fuel consumption. Under the circumstances, an antifouling paint based on a hydrolysis type resin has become the center of public attention. This type of coating composition is known as a polishing type antifouling paint. As the hydrolysis type resin, various polymers have been proposed as, for example, acrylic resins having electron attractive group bearing acyl bondings as halogenated aliphatic acid groups (e.g. Japanese patent application No. 101463/81 and ibid 198240/81); acrylic resins containing organic tin salts (e.g. Japanese patent application Kokai No. 98570/82); and polyester resins having metal ester bondings in the polymer main chain (e.g. Japanese patent application No. 165921/81 and ibid 165922/81) and the like. However, they were merely developed as resinous vehicles for antifouling paints, requireing film-forming properties and optimum film performance and therefore, there were in fact various limitations on the employable resins in respect of molecular weight, metal contents and the like, besides the hydrolysis natures thereof.
In a coating composition area, attempts have also been made to add, to a film-forming resinous varnish, resin powders for the improvement in application characteristics, without causing increase in viscosity, of the coating composition.
Therefore, even in an antifouling paint, hydrolysis type resin powders having no film-forming properties have been actually examined. For example, in Japanese patent publication No. 3830/86, are disclosed film-forming polymer compositions comprising a polyacrylic acid salt having a basic unit of the formula: ##STR1## in which M stands for Cu or Zn.
It is stated that said polyacrylic acid salts may be of film-forming type or of non-film-forming type and the molecular weight is in a range of 5000 to 1.times.10.sup.6.
Therefore, it is clear that hydrolysis type crosslinked resins having no film-forming properties are likewise suggested in this publication. However, in preparing said resins, a specific method is used therein, that is, a carboxyl bearing acrylic resin is first neutralized with caustic soda and dissolved in an aqueous medium and thus obtained polymer solution is reacted with a metal salt, thereby forming precipitation of insoluble polyacrylic acid salt. In this type of reaction, the soluble resin is gradually converted to insoluble type with the progress of ion-exchange reaction, and the formed insoluble resins are precipitated as amorphous masses each varying in size and shape. Since a smaller precipitate has a larger surface area and more rapidly hydrolyzed with sea water than a larger one, when the aforesaid precipitates are used in a self-polishing type antifouling paint, smaller precipitates are quickly hydrolyzed and consumed and larger precipitates are wastefully let out the coating with the dissolved resin.
Therefore, indeed an effective antifouling can be expected with the composition in an early stage, but a long-lasting effect cannot be obtained therewith. Furthermore, in the method of said Japanese patent publication No. 3830/86, an acrylic resin and a metal salt are reacted with each other each in aqueous solution form, and loss of solubility of the resin is the only cause of said precipitation. Since the reaction makes steady progress at the surface of precipitated resin interacted with aqueous metal salt solution, the metal ester bondings are always present in a higher concentration at the surface layers of the precipitates. Moreover, the precipitated resins do necessarily have a number of acid groups together with metal ester bondings, because precipitation is occured in an aqueous medium by the decrease in solubility of the resin. They are, therefore, too hygroscopic to use as the resinous filler in a polishing type antifouling paint. It is also pointed that it is quite difficult to control the metal ester containing crosslink density and carboxyl group content of the formed resin. For these reasons, a long-lasting antifouling effect cannot be expected with the coating composition added with the disclosed precipitates. Since the precipitates are not of spherical form, it is difficult to maintain them in a stabilized state of dispersion in a coating composition.
To the best of our knowledge, no antifouling coating compositions comprising hydrolysis type, insoluble polymer microparticles have been used so far.
It is, therefore, an object of the invention to provide a novel type of antifouling coating composition comprising a film-forming polymer, either hydrolysis type or non-hydrolysis type and either thermoplastic type or thermosetting type, an organic liquid diluent, an antifouling agent and other optional additives, added with polymer microparticles which are insoluble in the combination of said film-forming polymer and said organic liquid diluent, stably dispersed in the coating composition and thoroughly disintegrated from the interior and exterior of the respective particle in an ionic atmosphere, capable of resulting a coating with excellent polishing properties and long-lasting antifouling effects. An additional object of the invention is to provide an antifouling coating composition comprising disintegration type, crosslinked acrylic resin particles whose water susceptibility and hydrolysis rate are controllable each in appropriate optimum ranges at will. Other objects of the invention shall be clear from the descriptions of the specification and accompanied claims.